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Preliminary screening before crushing reduces the over crushing of the material, especially in rotor crushers. Curve 3 in Fig. 1 characterizes the fragment-size composition of the product of treating limestone’s in the SDA-1000 self-propelled crusher, which is fitted with an SMD-87 breaker unit and, unlike the AD-2 and SDA-300 systems, has a preliminary screening unit with an efficiency of 0.6 (a vibrating--grating system). When the product has a fragment size of 0-300 mm, we observe a more uniform distribution of fractional composition: The yield of the 0-25-mm class is only 26.3%. When the limestone is later subjected to final crushing (following the SDA-1000) in the crushing and sorting plant in KKD-900 and
Simons 5.5 cone breakers, also preceded by screening stages, the yield of the 0-25 mm fines.
in the end product of 0-80 mm is 38.5%, i.e., much less than from one-stage crushing in rotor crushers without preliminary screening (62.2%), but higher than for the cyclic technology in the same plant (33.8%).
The fact that the fines yield of the CFT system with SDA-IO00 machines is 4.7% greater than that for the CT in the same plant is due to over crushing during transportation by conveyers with five transfer points (2.2%, including 1.2% at one transfer point with a drop height of 8 m, the others being 2.5 m) together with over crushing in the SDA-1000 (2.5%) due to inefficient screening (0.6) and the relatively high peripheral velocity of the crushing rotor (20 m/sec). The yield could be reduced to the level given by the cyclic technology by reducing the drop height to 2.5 m and fitting screen feeders to the transfer points on the conveyer line, increasing the screening efficiency at the SDA-1000 (by partially re- designing the screens), and reducing the rotor speed of the SMD-87 crusher to 18 m/sec.
These recommendations were communicated to the Turgoyak Ore Administration who operates the SDA-1000 plant. The fragmentize composition of the end product and the yield of over- crushed material here, as well as the above factors, is largely governed by the fragment- size composition of the blasted rock. As a result of experimental investigations of the fragmentize composition of the limestone from blasting and mechanical crushing in the CFT system with an SDA-1000 plant, after mathematical processing we obtained the objective function for the yield of the 0-25 mm class in the end product of the crushing and screening plant:
where o- 25 Ye.pr is the yield of the 0-25 mm class in the end product; ~, screening efficiency of the SDA-1000; e, base of natural logarithms; dav, mean fragment diameter in the blasted rock; dav.ts, mean fragment diameter of the top screen product of the SDA-1000; and y~25, increase in the O-25-mm fines class during transportation along the conveyer line.
Solutions to this equation for various screening efficiencies at the SDA-1000 are plotted in Fig. 2. The graphs in Fig. 2 permit us to establish the rational fragment size for crushing limestone fluxes by blasting, expressed in terms of the weighted-mean fragment diameter dav at which the fines yield in the end product is an optimum or a minimum (shaded region).
If the screening efficiency at the SDA-1000 is 0.6, the rational fragment size gives a mean fragment diameter of 300 mm in the blasted rock. The existence of a rational fragment size in the blasted rock, such that mechanical crushing gives a minimum yield of fine fragments, is explained as follows. The fines yield increases when the fragment size of the blasted rock is less than the optimum value because of over crushing during blasting down. It also increases when the fragment size of the blasted rock is increased (~ = 0.7-0.9 in Fig. 2), because of over crushing of the large fragments in the first stage of mechanical crushing, beginning with partial deformation of the fragment in the form of fine spelling at its surface and formation of a large amount of fine fragments in this period.